Bis(adamantan-1-aminium) carbonate

In the title compound, 2C10H18N+·CO3 2−, the adamantan-1-aminium cation forms three N—H⋯O hydrogen bonds to three carbonate ions, resulting in a layer parallel to (001) with the adamantane groups located on its surface so that adjacent layers form only C—H⋯H—C contacts. The carbonate anions occupy special positions of 32 symmetry, whereas the adamantan-1-aminium cations occupy special positions of 3 symmetry.

In the title compound, 2C 10 H 18 N + ÁCO 3 2À , the adamantan-1aminium cation forms three N-HÁ Á ÁO hydrogen bonds to three carbonate ions, resulting in a layer parallel to (001) with the adamantane groups located on its surface so that adjacent layers form only C-HÁ Á ÁH-C contacts. The carbonate anions occupy special positions of 32 symmetry, whereas the adamantan-1-aminium cations occupy special positions of 3 symmetry.

Experimental
The University of the Witwatersrand and the Molecular Sciences Institute are acknowledged for providing the infrastructure required for this work. It has been reported that 1-aminoadamantane hydrochloride (marketed as Symmetrel) is effective in the prevention and treatment of the influenza (A) virus (Hoffmann, 1973;Dolin et al., 1982;Bright et al., 2005). However recent studies suggest that the virus is becoming increasingly resistant to this anti-influenza drug (Betakova, 2007).
In an attempt to crystallize pure 1-aminoadamantane from ethanol we obtained instead adamantan-1-aminium carbonate, illustrated in Fig. 1, suggesting that the amine had captured atmospheric CO 2 . We report the structure here. It is known that organic amines can trap CO 2 as the ammonium carbonate salt and this property is being explored as a way to capture carbon dioxide from the atmosphere (Yang et al., 2008).
Each carbonate ion of the title compound forms hydrogen bonds to six adamantane-ammonium ions, as shown in Fig. 2, forming a two-dimensional layer of adamantan-1-aminium carbonates parallel to (001). The hydrophobic adamantane layers interact with the neighbouring layers of adamantane-ammonium molecules via C-H···H-C contacts (see Fig. 3).
It is noted here that the structure of adamantan-1-aminium bicarbonate (Liu et al., 2009) reported in the literature is isomorphous to adamantan-1-aminium nitrate (Zhao et al., 2003). The former structure has unusually short H···H intermolecular contacts between NH 3 + group H atom and bicarbonate H atom of 1.50 Å In addition the geometry of the hydrogen carbonate ion is very similar to that of the nitrate ion. A re-investigation of these structures is warranted.

Experimental
Crystals were grown by slow evaporation of an ethanol solution of the title compound, 0.500 g in 10 ml of ethanol, and afforded colourless plates after three days under ambient conditions. Crystals decompose, with an emission of gas bubbles (presumably CO 2 ), at 423-428 K.

Refinement
The N-bound H atom was placed according to the observed electron density and was allowed to refine freely. The remaining H atoms were positioned geometrically and allowed to ride on their respective parent atoms, with C-H bond lengths of 1.00 (methine) and 0.99 Å (methylene CH 2 ) and with U iso (H) = 1.2 times U eq (C).

Computing details
Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT-Plus (Bruker, 2005); data reduction: SAINT-Plus and XPREP (Bruker 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997), Mercury (Macrae et al., 2008) and PLATON (Spek, 2009); software used to prepare material for publication: WinGX (Farrugia, 1999) and PLATON (Spek, 2009   The molecular structure of the title compound. Displacement ellipsoids are shown at the 50% probability level. The atoms C2f to C4f are generated by the symmetry (1-y,x-y,z); C2g to C4g by (1-x+y, 1-x,z); O1a by (-y,x-y,z) and O2b by (-x+y,-x,z).    Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.